Journal of Undergraduate Research
Volume 7, Issue 4 - March/April 2006

G protein β3 subunit (GNβ3) Polymorphism and Hypertension

M.A. Ortega, T. Y. Langaee, Ph.D., Y. Gong, Ph.D., H. Feng MD MS, J. A. Johnson, PharmD

ABSTRACT

Purpose

Hypertension (HTN) is a complex disorder and a major risk factor for coronary heart disease, stroke, and chronic renal failure. The GNβ3 gene encodes for the β3 subunit of heterotrimeric G protein that is involved in cell signaling. There have been conflicting data in the literature concerning the role of G protein β3 subunit (GNβ3) polymorphism (C825T) with increased risk for hypertension. In this study we investigated the association between GNβ3 C825T polymorphism and hypertension in African-American and Caucasian hypertensive (HT) and normotensive (NT) subjects.

Methods

A total of 654 subjects between 35 and 65 years old were analyzed for this study. Normotensive subjects were defined as having a sitting blood pressure (BP) < 140/90 mm Hg with no previous diagnosis and a family history of hypertension. Subjects were considered hypertensive if they had a family history of HTN and were diagnosed hypertensive and had a sitting BP ≥ 140/90 mm Hg, or taking antihypertensive medications. Genotyping for GNβ3 polymorphism was determined by polymerase chain reaction (PCR) followed by pyrosequencing. Logistic regression was used to model the probability of being hypertensive by race.

Results

Variant allele (T) frequency was 0.34 and 0.75 for Caucasians and African-Americans respectively. There was not significant association between GNβ3 genotypes and HTN in either African-Americans or Caucasians. After adjusting for age, gender, and BMI, the odds ratio and 95% confidence interval of being HT for TT vs CC, and TC vs CC genotypes were 1.227 (0.444, 3.391) and 0.975 (0.344-2.765) for African-Americans and 1.11 (0.504, 2.443) and 1.03 (0.619, 1.702) for Caucasians.

Conclusion

These results suggest that African-Americans and Caucasians with TT (the variant alleles) do not have a higher risk for developing hypertension.

INTRODUCTION

Hypertension (HTN) is a complex disorder and a major risk factor for coronary heart disease, stroke, and chronic renal failure. It is a multifactorial and polygenic disorder, dependent on an individual’s genetic background and various environmental factors. HTN occurs in over half of the U. S. population older than 65 years, and one in every three adults have high blood pressure.1 The GNβ3 C825T polymorphism has been studied extensively in the hypertension association studies.2

G proteins are composed of α, β, and γ subunits and are expressed in all human cells. The GNβ3 gene encodes for the β3 subunit of heterotrimeric G protein. The C825T polymorphism is located at nucleotide 825 in exon 10 and is linked with alternative splicing in exon 9 that results in deletion of 41 amino acids.3 The GNβ3 subunit has been found to be associated with increased intracellular signal transduction through the G-protein receptors.4 Polymorphisms in the G- protein subunit may have a significant effect on the cardiovascular system, influences in cell activation and cardiovascular responses to hormones. Siffert et al3 showed that the T allele of the polymorphism C825T allele causes a formation of a condensed protein with increased G-protein mediated signals that may elevate blood pressure by increased sodium-hydrogen exchanger activity, cytoplasmic calcium accumulation and increased production of smooth muscle cells.

There have been conflicting data in the literature concerning the role of G protein β3 subunit (GNβ3) polymorphism (C825T) with increased risk for hypertension. Various twin studies, pedigree analysis, and association studies reveal a genetic component to blood pressure and hypertension status in numerous populations.5 There is a fair amount of studies which associate the 825T allele with hypertension. The majority of studies performed in Caucasian populations confirm a positive association between the polymorphism and increased risk of hypertension as shown in a German population in the Belgium FLEMENGHO study.6 Dong et al7 conducted a study that for the first time shows a high frequency of the T allele may be a factor in developing hypertension in African Americans.7 In this study we chose to investigate the association between GNβ3 C825T polymorphism and hypertension in African American and Caucasian hypertensive (HT) and normotensive (NT) subjects.

METHODS

A total of 654 subjects, men and women between 35 and 65 years old, of African American and Caucasian descent, were obtained from the UF Center for Pharmcogenomics Hypertension database. In the study, 345 subjects were of Caucasian descent and 309 were of African American descent. All data concerning body mass index (BMI), race, age, and blood pressure (BP) was previously recorded and stored in the database. The information was not released until all the genotyping was done. Normotensive subjects were defined as having a sitting BP < 140/90 mm Hg with no previous diagnosis and no family history of hypertension in primary family members. Subjects were considered hypertensive if they had a family history of HTN and were diagnosed hypertensive and had a sitting BP ≥ 140/90 mm Hg, or were taking antihypertensive medications. Genomic DNA was isolated from mouthwash buccal cell samples by using the commercially available kits (PureGene, Gentra Systems Inc., Minneapolis, MN). Genotyping was determined by Pyrosequencing8 using a PSQ HS96A SNP reagent kit according to the manufacturer protocol (Biotage AB, Uppsala, Sweden). Genotyping for GNβ3 polymorphism C825T was determined by polymerase chain reaction (PCR), followed by pyrosequencing methods (Pyrosequencing, Uppsala, Sweden) at UF Center for Pharmacogenomics (UFCPGx). Valid genotyping was obtained in a total of 654 African American and Caucasian subjects and only those subjects were analyzed. Logistic regression was used to test the effect of genotype on the likelihood of hypertension while controlling the confounding factors. It modeled the probability of developing hypertension. Modeling used data from both races with a defined dominant model for the T allele. The confounding factors adjusted in the model include age, gender, and BMI. The statistical analyses were performed using the SAS version 9.1 (SAS Institute Inc, Cary, NC) statistical software package. Accordingly, the association between the presence of hypertension and genotype was presented as odd ratios (ORs) of TT and TC vs. CC (wildtype) genotypes.

RESULTS

The baseline demographics of the study participants from the hypertension database are shown in Table 1. The frequency for the T allele in the database is 0.53. The variant allele (T) frequency was 0.34 and 0.75 for Caucasians and African Americans respectively. Both hypertensive (HT) and normotensive (NT) African Americans have a higher T allele frequency than Caucasians. The frequency for HT African Americans, NT African Americans, HT Caucasians, and NT Caucasians is 0.77, 0.73, 0.35, and 0.32 respectively. No significant differences existed in age and sex adjusted characteristics by genotype. The results of the Logistic regression model confirmed no significant association between hypertension and the T allele for either race.

Table 1
Demographic Characteristics for Hypertension Database
Characteristics Total subjects
(n=654)		 NT-Caucasian
(n=164)		 HT-Caucasian
(n=181)		 NT-African American
(n=122)		 HT-African American
(n=187)
Age 49.1 ± 8.2 47.7 ± 8.3 51.9 ± 7.6 45.1 ± 7.2 50.4 ± 8.1
Male 44% 44$ 55% 47% 29%
BMI, kg/m2 30.3 ± 6.9 26.5 ± 5.4 31.4 ± 6.6 30.2 ± 6.8 32.7 ± 6.9
SBP, m Hg 129.3 ± 18.8 117.1 ± 12.2 136.4 ± 18.9 118.6 ± 9.0 140.0 ± 18.4
DBP, mm Hg 81.6 ± 11.8 74.9 ± 8.0 84.6 ± 13.5 76.8 ± 6.2 87.8 ± 11.5
T allele frequency 0.53 0.32 0.35 0.73 0.77
Mean ± SD

 

The frequencies of genotypes CC, CT, and TT by race are shown in Figure 1. The observed frequencies for the genotypes CC, CT, and TT for African Americans 6.47%, 36.3%, 57.3%, and for Caucasians 44.9%, 42.0%, 13.0%, respectively . African Americans have a much greater frequency of the T allele in comparison to Caucasians. A higher prevalence of hypertension was seen among the carriers of the T variant both as heterozygotes and homozygotes compared with the CC genotype in African Americans. In addition, the blood pressures tended to only be slightly higher in those with the 825T variant (Figure 2). The average systolic blood pressure (SBP) and diastolic blood pressure (DBP) in mmHg only slightly differ among genotypes and not statistically significant. The SBP and DBP for CC, CT, and TT alleles were 127.4, 128.5, 131.7 and 80.6, 81.5, 82.8 respectively. After adjusting for age, gender, and BMI, the odds ratio and 95% confidence interval of being hypertensive for TT vs. CC, and TC vs. CC genotypes were 1.375(0.87-2.14) and 0.99 (0.656-1.521) respectively. The odds ratio for African Americans and Caucasians for TT vs. CC, and TC vs. CC genotypes were 1.227(0.444,3.391), 0.975(0.344,2.765), 1.11(0.504,2.443), and 1.03(0.619,1.702), correspondingly. No significance was found in African Americans or Caucasians, as shown in Figure 3.

Figure 1. Genotype distribution of GN≤3 among African Americans and Caucasians
Figure 1. Genotype distribution of GNβ3 among African Americans and Caucasians

 

Figure 2. Association of GN≤3 genotypes and BP
Figure 2. Association of GNβ3 genotypes and BP

 

Figure 3. Genotype Odds Ratio of GN≤3 in African American and Caucasian populations

Figure 3. Genotype Odds Ratio of GNβ3 in African American and Caucasian populations

DISCUSSION

There was no significant association between GNβ3 genotypes and HTN in African Americans and Caucasians in our hypertension database. These results suggest that African Americans and Caucasians with TT (the variant alleles) do not have a higher risk for developing hypertension. A review of hypertension studies involving the polymorphism to be associated in Caucasians and African Americans reveals a lot of conflicting results. Our study alone differs greatly in comparison with the literature. In comparison with the Siffert et al.3 and the FLEMENGHO study, we did not find an association between GNβ3 genotypes and HTN in Caucasians. The former studies confirmed a positive association between the polymorphism and increased risk of hypertension, but we found no association in Caucasians.

Dong et al.7 showed a high frequency of the T allele may be a factor in developing hypertension in African Americans. On the contrary, our findings support the literature of Rotimi et al.10 suggesting there is a lack of association in African American populations with the T allele and hypertension.9,10

The results of the association or no association with HTN seem to depend highly on the populations’ ethnicity. According to Siffert et al.3, Caucasian population’s genotypes of the C825T allele are roughly distributed as 45% CC, 45% TC, and 10% TT, almost about the same frequencies we observed. However, we did not find any positive association between hypertension and the T allele in Caucasians in comparison to other studies.3,6 Siffert et al.3 also noted that old ethnicities such as African American show high frequencies of the T allele, also what we observed.4 Contrary to our preliminary results and analysis presented as a poster, when we further analyzed our genotyping data and confirmed some of the genotyping with another genotyping method, we found no association between this polymorphism and HTN.

In summary, there was no significant association between GNβ3 genotypes and HTN in African Americans and Caucasians. This further provides evidence that the 825T allele may not be a factor for the development of HTN even though it has been reported to enhance the activation of the G-proteins. Our study along with others in the literature shows no significant association with HTN. Despite the lack of association between hypertension and polymorphism, the high TT frequency further exacerbates the factor of interindividual differences.

REFERENCES

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